Methods for channel-dependent time-and-frequency-domain scheduling and related communication nodes

ABSTRACT

In a method of improved channel-dependent time- and frequency-domain scheduling in an OFDM based telecommunication system with multiple user terminals, determining SO a parameter value representative of the system load; pre-selecting S1 a subset of user terminals if the determined parameter value is larger than or equal to a predetermined threshold; and performing S2 frequency-domain scheduling of the pre-selected subset, to reduce the downlink signaling overhead and enabling improved efficiency of the channel-dependent time- and frequency-domain scheduling.

TECHNICAL FIELD

The present invention relates to telecommunication systems in general,and specifically to methods and arrangements for improvedchannel-dependent time-and-frequency-domain scheduling in an OrthogonalFrequency Division Multiplex (OFDM) based telecommunication system.

BACKGROUND

In the area of user resource allocation for wireless fading channels,great effort is presently focused on scheduling [1-2]. Schedulingalgorithms can typically be classified into channel-dependent orchannel-independent scheduling according to the dependence on thechannel. An example of such an algorithm is Round Robin (RR), a typicalchannel-independent scheduling, which benefits from simplicity at theprice of poor performance. The class of channel-dependent schedulingalgorithms utilize the so-called channel state information (CSI) orchannel quality indicator (CQI) in order to improve the systemperformance.

For OFDM systems, the above mentioned channel-dependent scheduling canbe further classified into so called time-domain scheduling, where asingle user or user terminal per frame is scheduled in a given timescale, and so called time-and-frequency-domain scheduling, wheremultiple users per frame are scheduled exclusively in a given timescale. The time-and-frequency-domain scheduling, hereinafter referred toas frequency-domain scheduling, has previously been shown to providebetter performance than the time-domain scheduling due to the multi-userdiversity in the frequency domain, especially for wideband transmissions[1]. However, the frequency-domain scheduling requires CSI or CQIfeedback once per frequency-domain resource unit, which requiresextensive overhead signaling that is much higher than that fortime-domain scheduling, i.e. one feedback for the whole band at a time.In addition, there are many different detailed criteria for thefrequency-domain scheduling, such as Max-CIR, Proportional-Fair (PF),weighted-queue-PF etc [3], for both frequency-domain and time-domainscheduling.

For the class of channel-dependent scheduling algorithms the time-domainscheduling has the advantages of low computational complexity and lowsignaling overhead (for it self and power allocation, link adaptationafterwards as well). However, due to the frequency-selectivity along thewideband, the time-domain scheduling cannot guarantee that the scheduleduser performs well on the whole band, therefore, can hardly achieve goodperformances in capacity and coverage.

Frequency-domain scheduling schemes perform the criteria in the morerefined sub-group (e.g. chunk) of the whole band, and utilize themulti-user diversity as well, so that the performances in capacity andcoverage are greatly improved as compared to the time-domain schedulingschemes.

However, the disadvantages of the otherwise advantageousfrequency-domain scheduling increase as the performance improves.Specifically, the computational complexity increases greatly with thenumber of chunks and the system load. In addition, since the scheduleduser terminals may be different from one chunk to another, a largequantity of DL signaling is required for the frequency-domain adaptation(FDA), including the chunk allocation and the subsequent powerallocation and link adaptation per user. The signaling overhead thusincreases linearly with the increasing bandwidth, i.e. with the numberof chunks, and with the system load, i.e. the number of users.

These disadvantages have prevented, up until now, the furtherexploitation of channel-dependent time-and-frequency domain scheduling.

Consequently, there is a need for methods and arrangements enablingexploiting the advantages of channel-dependent time- and frequencydomain scheduling whilst at the same time reducing the knowndisadvantages.

SUMMARY

A general problem with known channel-dependent scheduling algorithms ishow to utilize the advantageous performance of frequency domainscheduling but without the above described disadvantages.

A general object of the present invention is to provide methods andarrangements for improved channel-dependent frequency-domain scheduling.

These and other objects are achieved according to the attached set ofclaims.

According to a basic aspect, the present invention comprises determiningthe load of the system. If the load equals or surpasses a predeterminedthreshold value, a subset of all the user terminals are pre-selected forscheduling and subsequently scheduled.

Advantages of the present invention comprise:

-   -   Reduced overhead downlink signaling for channel-dependent        frequency-domain scheduling.    -   Reduced overhead signaling for resource allocation.    -   Reduced overhead signaling for link adaptation    -   The complexity of the corresponding frequency-domain adaptation        can be limited within a pre-defined scale.    -   The scales of signaling overhead and computational complexity        can be pre-defined as fixed, regardless of varying system load,        for the given bandwidth, which is favoured by the further        signaling design.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by referring to the following description takentogether with the accompanying drawings, in which:

FIG. 1 is a schematic flow diagram of an embodiment of a methodaccording to the invention;

FIG. 2 is a schematic illustration of an embodiment of an arrangementaccording to the invention;

FIG. 3 shows the impact for a first cell load of the implementation ofan embodiment of the method and arrangement according to the invention;

FIG. 4 shows the impact for a second cell load of the implementation ofan embodiment of the method and arrangement according to the invention.

ABBREVIATIONS

-   BPSK Binary Phase Shift Keying-   CIR Carrier to Interference Ratio-   CQI Channel Quality Indicator-   CSI Channel State Information-   DL Down Link-   FDA Frequency Domain Adaptation-   HSDPA High-Speed Downlink Packet Access-   LA Link Adaptation-   MCS Modulation and Coding Scheme-   OFDM Orthogonal Frequency Division Multiplex-   PA Power Allocation-   PF Proportional Fair-   PFT Proportional Fair in the Time domain-   PFTF Proportional Fair in the Time and Frequency domain-   QAM Quadrature Amplitude Modulation-   QPSK Quadrature Phase Shift Keying-   RR Round Robin-   SINR Signal to Interference Noise Ratio

DETAILED DESCRIPTION

To provide a further insight into the disadvantages of channel-dependentfrequency-domain scheduling as compared to channel-dependent time-domainscheduling a further detailed discussion and analysis is provided below.

The frequency-domain adaptation (FDA) related signaling overhead in thedownlink (DL) consists of the signaling used to inform FDA decisions foreach user or user terminal, where the pilot signals for signal tointerference noise ratio (SINR) estimation are assumed to be the samefor all the schemes, therefore not taken into account in the following.

As an illustrative example, consider a system with N_(ch) chunks in theDL to serve N users per cell, where the number N_(bitsDCCH) of signalingbits per frame is calculated according to the following:

Time-Domain Scheduling: (Round Robin (RR) or Proportional-Fair (PF) inTime Domain)

-   -   For user index with uniform power allocation (PA): 1×┌ log₂        N┐bits/frame, where ┌x┐ is the minimum integer that is equal to        or larger than x.    -   For user index with On/Off PA: 1×┌ log₂ N┐+1×N_(ch) bits/frame    -   For link adaptation (LA): k_(mod)+k_(cr) bits/frame, e.g. for 20        MHz bandwidth        -   Modulation mode index among {BPSK, QPSK, 16QAM, 64QAM}:            k_(mod)=2 bits/frame        -   Index for coding rate, corresponding to information block            size: k_(cr)=f(N_(symsperframe), ModOrder) (e.g. at most 19            bits/frame for 20 MHzm while for HSDPA it is about 6            bits/frame)

For the example for RR time-domain scheduling, uniform power allocationand single modulation and code scheme (MCS) with continuous coding rate,the considered number of DL signaling bits N_(bitsDCCH) becomes:N _(bitsDCCH)=┌ log₂(N)┐+(k _(mod) +k _(cr))[bits/frame],  (1)Frequency-Domain Scheduling

-   -   For user index per chunk with uniform PA: N_(ch)×┌ log₂ N┐        bits/frame;    -   For user index per chunk with On/Off PA: N_(ch)×┌ log₂        N┐+N_(ch)×1 [bits/frame]    -   For LA: f(N)=k_(mod)·N+k_(cr)·N bits/frame, depending on how        many users are to be scheduled    -   For the example of Proportional Fair in Time and Frequency        domain (PFTF), uniform PA and single MCS with continuous coding        rate, the total DL FDA signaling N_(bitsDCCH) is determined        according to:        N _(bitsDCCH) =N _(ch)┌ log₂(N)┐+N×(k _(mod) +k        _(cr))[bits/frame]  (2)    -    which is substantially larger than that of time-domain        scheduling. As can be seen in Equation (2) the scheduling        algorithm is the largest contributor to the DL overhead        signaling.

Accordingly, as recognized by the inventors, if the DL signaling of FDAcan be reduced, the efficiency and usefulness of channel-dependentfrequency-domain scheduling can be further improved.

A basic embodiment according to the invention thus provides a method ofpre-selecting a subset of the active user terminals in a system andperforming frequency-domain scheduling, link adaptation and resourceallocation for that subset. The remaining set of user terminals areprocessed subsequently, thereby significantly reducing the DL overheadsignaling in each time instance.

A specific embodiment of a method according to the invention will bedescribed below with reference to the schematic flow diagram of FIG. 1.Initially, a present load of the system is determined S0 and compared toa preset threshold. The load can be determined by measuring some measureor parameter value that is dependent on the total load, i.e., relativesignaling overhead for all active users. Consequently, according to aspecific embodiment, the above mentioned present threshold is a specificvalue of the signaling overhead, i.e., 10%.

If the determined load of the system is larger than or equal to thepredetermined threshold, pre-selection is deemed necessary, and a subsetof user terminals are pre-selected S1 for scheduling. The subset of userterminals comprises at least two user terminals and less than allterminals.

Finally, the subset of user terminals is subjected to scheduling S2 andoptionally link adaptation and resource allocation according to knownmeasures.

If the threshold is not surpassed, then all user terminals are scheduledin a known manner.

According to a specific embodiment, the pre-selection process canoptionally be repeated for a plurality of subsets until the measuredload of the system is below the preset threshold or based on some othercriteria.

There are several potential criteria for pre-selecting the abovementioned subset of user terminals:

-   -   Random selection: pre-select the user terminals randomly;    -   Max-CIR selection: according to a Max-CIR criterion

$\begin{matrix}{{J = {\arg{\max\limits_{1 \leq n \leq N}\gamma^{(n)}}}},} & (3)\end{matrix}$where γ^((n)) denotes the estimated SINR of user n, the user(s) with thehighest SINR(s) according to Equation (3) are selected frame-wise orchunk-wise.

-   -   PF-based selection: according to the PF criterion

$\begin{matrix}{J = {\arg{\max\limits_{1 \leq n \leq N}{\frac{{TP}_{{cs}\; 1}^{(n)}}{{TP}_{av}^{(n)}}.}}}} & (4)\end{matrix}$where TP_(est) ^((n)) denotes the estimated throughput of user terminalwith index n and TP_(av) ^((n)) stands for the average throughput ofuser terminal n each frame, the user(s) with the highest ratio(s) of (4)are selected frame-wise.

There may also be other pre-selection criteria based on chunk-wise PF orincluding other cost functions or quality of service (QoS) for each userterminal.

By utilizing the step of pre-selection, the users are limited to apre-defined scale in order to reduce the corresponding DL signaling.

Among the pre-selected users, the frequency-domain scheduling, powerallocation and link adaptation can be further performed. The non-electedor discarded user terminals can optionally be queued until the nextround of signal processing. In this manner the number of bits for the DLFDA signaling of Equation (2) for the simplified PFTF schemes becomesN _(bitsDCCH) =N _(sel)×┌ log₂(N)┐+N _(ch)×┌ log₂(N _(sel))┐+N _(sel)×(k_(mod) +k _(cr))[bits/frame]  [5]where the first term of Equation (5) corresponds to the user index ofthe pre-selected user terminals.

To illustrate the benefits of the method according to the inventionfurther, a few simulation experiments are presented and described below.

The basic simulation parameters are summarized below in Table 1.

TABLE 1 Basic simulation parameters Cell Plan Number of sites 7Sectors/site 1 Frequency reuse 1 Cell radius [meter] {500, 1000, 2000,3000} System Duplex mode FDD Assumption Channel model 3GPP SCME,Suburban macro Bandwidth [MHz] 20 Available subcarriers/ 1280 tonesChunk size 16 [tones per chunk] Traffic model Full-buffer traffic modelTransmitter/receiver SISO, omni antennas Transmission 80, uniform powerallocation power [watt.] Average offered 8, 30 calls per cellTransmission Modulation and coding Single MCS per frame schemes scheme(MCS) Modulation BPSK, QPSK, 16QAM, 64QAM Link adaptation (LA)BLER_target = 0.1 Coding rate Continuous Remarks: (*) the same MCS forall chunks allocated to one user in an OFDM frame

For the simulations the Priority-Fair (PF) criteria in the time-domain(PFT) scheduling represents the channel-dependent time-domainscheduling. The PF in time and frequency domain (PFTF) represents thechannel-dependent frequency-domain scheduling. For pre-selectionschemes, random selection and PFT selection are considered. Therefore,in the following description four schemes are compared forillustrations, namely:

-   -   Pure time domain scheduling (PFT),    -   Frequency-domain scheduling (PFTF) with Random pre-selection        (Rand+PFTF),    -   Priority-Fair (PFT) pre-selection in time domain+PFTF (PFT+PFTF)        and    -   Frequency-domain scheduling (PFTF) without pre-selection

Two cases of different system load e.g. 8 and 30 user terminals areconsidered. However, the invention is not limited to those loadscenarios. Moreover, for the load of 30 user terminals the impact of thepre-selection bound (4 and 8) is also shown. The DL signaling of theabove schemes under different cases are listed below in Table 2.

TABLE 2 DL signaling of the four schemes User pre-selection + PFTF PFTFCell load Overhead Relative Overhead Relative [users/cell] [bits/frame]overhead [bits/frame] overhead 8 256 (bound 4)   5% 408  8% 30 264(bound 4) 5.1% 1030 20% 448 (bound 8) 8.7%

It can be seen that with any type of user pre-selection, the resultingDL signaling is fixed and much less than the pure frequency-domainscheduling PFTF. Especially in the case of high load, e.g., 30, the DLsignaling with user pre-selection is even smaller than the pure PFTFscheme. The reduction ratios (to the signaling of the pure PFTF) are 74%(bound of 4) and 57% (bound of 8), respectively, which are veryremarkable.

It also implies that the resulting computational complexity is greatlyreduced by the application of user pre-selection according to theinvention.

On the other hand, the slight performance loss as the price of signalingrejection deserves observation. FIGS. 3 and 4 depict the performances ofthe aforementioned schemes in the average cell throughput and the 5^(th)percentile user throughput.

It can be seen that the PFT+PFTF scheme shows a little worse performancethan the pure PFTF. Especially in the high load case, the PFT+PFTFscheme with bound of 8 shows very close performance to the pure PFTF.

Of course, there are other options for user pre-selection, which mighthave even better performance than the ones shown. Therefore, userpre-selection provides the potential to further improve the systemperformance with limited DL signaling.

A node according to an embodiment of the present invention is configuredfor enabling the above discussed method according to the invention, andwill be described with reference to FIG. 2. The node can, according to aspecific embodiment, be represented by but not limited to a Node B in aUMTS telecommunication network.

A basic embodiment of a node 1 according to the invention comprises aload determination unit 10 which enables measuring or at least acquiringa measure of the present load in the system, a comparing unit 11 thatcompares the load measure to a preset load measure threshold, and apre-selection unit 12 that pre-selects a subset of user terminals forscheduling if the load measure surpasses the threshold value. Finally,the node 1 comprises a scheduling unit 13 for scheduling thepre-selected user terminals.

If the threshold is not surpassed by the present load, the schedulingunit 13 is adapted for scheduling all user terminals in a known manner.

Advantages of the Invention comprise:

-   -   The DL signaling overhead, which is related to channel-dependent        frequency-domain scheduling is greatly reduced by limiting the        number of users for scheduling;        -   Less signaling overhead is required by frequency-domain            resource allocation due to the reduced amount of users;        -   Less signaling overhead is required by link adaptation due            to the reduced amount of users;    -   The complexity of the corresponding frequency-domain adaptation        can be limited within a pre-defined scale;    -   The scales of signaling overhead and computational complexity        can be pre-defined as fixed, regardless of varying system load,        for the given bandwidth, which is favored by the further        signaling design.

It will be understood by those skilled in the art that variousmodifications and changes, including combinations of variousembodiments, may be made to the present invention without departure fromthe scope thereof, which is defined by the appended claims.

REFERENCES

-   [1] R. Knopp and P. Humblet, Information capacity and power control    in single-cell multiuser communication, in Proc. of Int. Conf. on    Communication, June 1995, vol. 1, pp. 331-335.-   [2] W. Anchun, X. Liang, Z. Shidong, X. Xibin, and Y. Tan, Dynamic    resource management in the fourth generation wireless systems,    Proceedings IEEE International Conference on Communication    Technology, April 2003.-   [3] K. Muhammad, Comparison of Scheduling Algorithms for WCDMA    HS-DSCH in different Traffic Scenarios, IEEE PIMRC'2003 Beijing,    China 2003.

The invention claimed is:
 1. A method of improved channel-dependenttime- and frequency-domain scheduling in an OrthogonalFrequency-Division Multiplexing (OFDM) based telecommunication system,said system comprising a plurality of user terminals, the methodcomprising: determining at a node of a telecommunication network aparameter value representative of a system load; responsive todetermining the parameter value representative of the system load,pre-selecting at the node of the telecommunication network a subset ofsaid plurality of user terminals, the subset being limited at the nodeof the telecommunication network to a pre-defined scale, when saiddetermined parameter value representative of the system load is largerthan a predetermined threshold; and responsive to determining theparameter value representative of the system load, performing at thenode of the telecommunication network frequency-domain scheduling ofsaid pre-selected subset of user terminals when said determinedparameter value representative of the system load is larger than thepredetermined threshold to reduce the downlink signaling overhead and toenable improved efficiency of the channel-dependent time- andfrequency-domain scheduling.
 2. The method according to claim 1,comprising repeating the determining, pre-selecting and frequency-domainscheduling for the remaining user terminals.
 3. The method according toclaim 1, where said determined parameter value representative of systemload comprises a relative signaling overhead for all user terminals ofthe plurality of user terminals so that pre-selecting a subset of saidplurality of user terminals comprises pre-selecting the subset of saidplurality of user terminals at the node of the telecommunication networkwhen said relative signaling overhead is larger than a predeterminedthreshold.
 4. The method according to claim 3, where said predeterminedthreshold comprises a relative signaling overhead of 10%.
 5. The methodaccording to claim 1, wherein pre-selecting a subset comprisespre-selecting said subset randomly from said plurality of user terminalsat the node of the telecommunication network.
 6. The method according toclaim 1, wherein pre-selecting a subset comprises pre-selecting saidsubset based on a maximum Carrier to Interference Ratio (Max-CIR)criterion at the node of the telecommunication network.
 7. The methodaccording to claim 6, wherein pre-selecting a subset comprisespre-selecting user terminals with the highest signal to interferencenoise ratio at the node of the telecommunication network.
 8. The methodaccording to claim 7, wherein pre-selecting a subset comprisespre-selecting said user terminals frame-wise and chunk-wise at the nodeof the telecommunication network.
 9. The method according to claim 1,wherein pre-selecting a subset comprises pre-selecting said subset basedon a ratio between an estimated throughput and an average throughput foreach user terminal at the node of the telecommunication network.
 10. Themethod according to claim 9, wherein pre-selecting a subset comprisespre-selecting user terminals with the highest ratio at the node of thetelecommunication network.
 11. The method according to claim 1, whereinpre-selecting a subset comprises pre-selecting said subset based on oneor a combination of throughput and/or quality of service.
 12. The methodaccording to claim 1, where said subset of user terminals comprises atleast two user terminals and not all user terminals.
 13. The methodaccording to claim 1 wherein the pre-defined scale is fixed.
 14. Themethod according to claim 1 further comprising: responsive todetermining the parameter value representative of the system load,performing at the node of the telecommunication network scheduling ofall user terminals when said determined parameter value representativeof the system load is less than the predetermined threshold.
 15. A nodein an Orthogonal Frequency-Division Multiplexing (OFDM) communicationsystem, said system comprising a plurality of user terminals, said nodecomprising: a load determinator configured to determine a parametervalue representative of a system load; a pre-selector configured topre-select a subset of the plurality of user terminals, the subset beinglimited at the node to a pre-defined scale, responsive to determiningthe parameter value representative of the system load when saiddetermined parameter value is larger than a predetermined thresholdvalue; and a scheduler configured to perform frequency-domain schedulingof said pre-selected subset of user terminals responsive to determiningthe parameter value representative of the system load to reduce thedownlink signaling overhead and to enable improved efficiency ofchannel-dependent frequency domain scheduling.
 16. The node according toclaim 15 wherein the pre-selector and scheduler are configured to repeatoperations of pre-selection and scheduling for the remaining userterminals.
 17. The node according to claim 15, where said pre-selectoris further configured to pre-select said subset of user terminals at thenode based on one or a combination of signal to interference noiseratio, randomly, maximum Carrier to Interference Ratio (Max-CIR),quality of service, throughput.
 18. The node according to claim 17,where said node is a Node B in a Universal Mobile TelecommunicationsSystem (UMTS).
 19. The node according to claim 15, where said node is aNode B in a Universal Mobile Telecommunications System (UMTS).
 20. Thenode according to claim 15, where said determined parameter valuerepresentative of system load comprises a relative signaling overheadfor all user terminals of the plurality of user terminals so that thepre-selector is configured to pre-select a subset of said plurality ofuser terminals by pre-selecting the subset of said plurality of userterminals if when said relative signaling overhead is larger than apredetermined threshold.
 21. The node according to claim 15 wherein thescheduler is further configured to perform scheduling of all userterminals responsive to determining the parameter value representativeof the system load when said determined parameter value representativeof the system load is less than the predetermined threshold.
 22. Thenode according to claim 15, wherein the pre-selector is configured topre-select the subset by pre-selecting user terminals with the highestsignal to interference noise ratio frame-wise and chunk-wise.